Sphinx of Dilemmas

Flying, Prowess
{1}{u}{u}: Monstrosity 1
When Sphinx of Dilemmas becomes monstrous, reveal the top 5 cards of your library. Your opponent separates them into two piles, face down and you choose one of those piles. Keep the chosen pile, and put the rest into your graveyard.
There are two sisters: one gives birth to the other and she, in turn, gives birth to the first....